1. Field of the Invention
The present invention relates to sewing machines, and more particularly, to pull rollers which are used for pulling fabric past a sewing needle.
2. Description of the Prior Art
When sewing some types of soft and elastic fabric materials with traditional sewing machines, a common problem which develops is the forming of wrinkles on the fabric. This problem long impeded progress in the garment industry in upgrading the quality of sewing, since bunching or wrinkling can occur while using even high quality fabric materials. In the past, domestic sewers in the home have solved the wrinkling problem by tensioning the fabric material to be sewed by pulling it with their hands. Until recently, this solution was not practical on a mass production basis. A few years ago, a puller device was developed for a sewing machine wherein a pull roller was used to maintain tension on the fabric while it was being sewn.
FIG. 1 illustrates one type of pull roller arrangement that is known in the art. A sewing machine having a needle F and a presser foot A is provided with a support mechanism for a pull roller B.sub.1. A power rotary shaft C is provided for turning the pull roller B.sub.1. In operation, a piece of material to be sewn is pinched between pull roller B.sub.1 and a driven roller D so that tension is maintained in the material along a distance G between the rollers and the needle F.
Although the apparatus shown in FIG. 1 was effective to some degree in eliminating the wrinkling problem, there were also disadvantages in the design, examples of which are:
1. The relatively long distance G between the point of application E of the pull roller B.sub.1 to the material to be sewn and the sewing needle F allows the tensioned portion of the material to expand relative to surrounding portions thereof during sewing, resulting in an inferior quality of garment.
2. The relatively long distance G between the point of application E of the pull roller B.sub.1 to the material to be sewn and the sewing needle A also makes it difficult to turn the fabric while sewing.
3. Also owing to the relatively long distance G between the point of application E and the sewing needle F, small pieces of material such as labels cannot be tensioned while they are being sewn to an underlying material.
FIG. 2 illustrates another type of pull roller assembly that is known in the prior art. As in the apparatus of FIG. 1, a sewing needle F is provided on a sewing machine along with a presser foot A. A relatively small pull roller B.sub.2 is provided to tension the material to be sewn. As in the apparatus of FIG. 1, the pull roller B.sub.2 in FIG. 2 is mounted for linear movement, although pull roller B.sub.2 can be mounted much closer to the sewing needle F because of it size and because of a recess A.sub.1 that is provided in the heel of the presser foot A. Although the prior art apparatus illustrated in FIG. 2 solved many of the problems that were present in the apparatus of FIG. 1, its design created additional problems, most of which are:
1. The positioning of the pull roller B.sub.2 in the recess A.sub.1 in the heel of the pressure foot limited the distance the pull roller could be lifted above the material to be sewn. Thus, it was impossible to completely disengage the pull roller B.sub.2 from the material to be sewn when sewing an especially thick material.
2. Since the pull roller B.sub.2 was disposed within the recess A.sub.1 in the presser foot, it was difficult to completely disengage the pull roller B.sub.2 from material that did not need to be tensioned while it is being sewn. For that reason, the apparatus illustrated in FIG. 2 had utility only for sewing certain types of soft or elastic materials.
FIG. 3 illustrates another type of prior art material tensioning apparatus. A sewing machine having a sewing needle F and a presser foot A is provided with a recess in the heel A.sub.1 of the presser foot and an inclined linear mounting assembly for a pull roller B.sub.3. The pull roller B.sub.3 is driven downwardly by means of a spring biased gliding rod H toward a worktable K. A plate I is provided on the mounting assembly so that the pull roller B.sub.3 is automatically lifted from the worktable K when the presser foot A is moved upwardly. In addition, the angle of inclination of the mounting means H for the pull roller B.sub.3 prevents the pull roller from abutting the heel A.sub.1 of the presser foot while it is being withdrawn from the material. Although the apparatus illustrated in FIG. 3 solved many of the problems that were presented in the devices in FIGS. 1 and 2, its design created new problems, examples of which are:
1. During sewing, it will sometimes be desirable to adjust the angle of inclination .theta. between the axis of the driving rod H and the plane of the worktable K. However, when this is done, the force necessary to raise the presser foot A also changes. When .theta. is small, it takes relatively little force to lift the presser foot. However, when .theta. is large, it takes a great deal more force to lift the presser foot A. This increases the difficulty of operation of the apparatus, as well as decreasing the quality of the garment being sewn. A vector analysis of this problem is illustrated in FIGS. 4 and 5. In those Figures, P.sub.1 is the force of application; R.sub.1 and R.sub.2 are the reaction forces exerted by the mounting assembly on the driving rod H; F.sub.1 and F.sub.2 represent the frictional forces generated by the reaction force; F.sub.3 represents the frictional force generated between the presser foot A and the plate I, and S represents the spring biasing force that is exerted on the driving rod H. From FIGS. 4 and 5, it is evident that in order to raise the presser foot, P.sub.1 .theta. must be greater than S+F.sub.1 +F.sub.2 +F.sub.3 COS .theta.. Thus, the larger .theta. becomes, the greater the lifting force P.sub.1 is needed to lift the presser foot A.
2. The frictional forces F.sub.1 and F.sub.2 which are generated between the driving rod H and the mounting assembly themselves generate a moment of force which tends to rock the pressure foot in its bearings and increases the difficulty in raising the pressure foot.
3. The rightward component of force that is exerted by the puller roller P.sub.3 on the material, as illustrated in FIG. 4, can cause the material to bunch up in front of the roller, which defeats the purpose of the apparatus.
Clearly, there exists a long and unfilled need in the prior art for a puller roller assembly for a sewing machine without the above discussed disadvantages.